1. Field of the Invention
This invention relates to control valves and more particularly to parking brake control valves suitable for use in tractor-trailer combinations.
2. Description of the Prior Art
In recent years, with the advent of improvements in vehicle braking systems, dual supply brake systems have come into use. These systems often employ two "dry" tanks, supplied through check valves from a common "wet" tank; each of the dry tanks constitutes the source of an independent supply line for the vehicle braking system. Heretofore, in some prior art systems a two-way check valve connected to both of the independent supply lines has been employed to provide communication, through a single line connected to the output of the check valve, from the supply lines to a manually operable control valve in the cab of the tractor. This latter valve controls the application of supply pressure to the spring chamber of each of one or more spring brake actuators (of a type such as disclosed in U.S. Pat. No. 3,908,520 issued to J. W. Ma on Sept. 30, 1975) which are employed on the tractor and trailer.
When supply pressure greater than a minimum amplitude is communicated by the control valve to the spring chambers, brake application is controlled by the service brake chamber of these actuators under the influence of service pressure, as modulated by the driver's foot pedal. When, however, the control valve provides ambient or exhaust pressure to the spring chambers, springs contained therein activate the brakes irrespective of the service pressure. Exemplary of known control valves are those disclosed in the following patents: U.S. Pat. No. 3,027,905 issued to H. L. Dobrikin on Apr. 3, 1962; U.S. Pat. No. 3,168,353 issued to C. Horowitz on Feb. 2, 1965; U.S. Pat. No. 3,740,106 issued to C. Horowitz on June 19, 1973; U.S. Pat. No. 3,759,584 issued to B. Klimek on Sept. 18, 1973; U.S. Pat. No. 3,237,995 issued to H. L. Dobrikin on Mar. 1, 1966; U.S. Pat. No. 3,692,366 issued to S. Campanini on Sept. 19, 1972.
The link between the aforementioned two-way check valve and the control valve is a non-duplicated (non-redundant) link. Failure of this single line would effectively defeat the purpose of the dual brake supply arrangement above-described and the prospect of such failure impairs the usefulness of such systems.